What is dark matter?

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Physics is unique in the scientific world, in that its reliance on math means it can come to a broad consensus on matters with very little evidence available. In Earth science, a veritable mountain of evidence can’t fully bury the issue of global warming, and even with the vast majority of scientists now convinced, a vocal minority still dissent. Yet in the case of physics and dark matter, a substance defined as being virtually immune to observation, there are no meaningful dark matter deniers left standing. So what is dark matter, and how has physics come to such a powerful agreement on the idea that it makes up the vast majority of matter in the universe?

Matter, the regular kind that makes up the atmosphere, the Sun, Pluto, and Donald Trump, interacts with the universe in a number of ways. It absorbs, and in many cases emits, electromagnetic radiation in the form of gamma rays, visible light, infra-red, and more. It can generate magnetic fields of various sorts and strengths. And matter has mass, creating the force of gravity, the effects of which can be readily observed. All these things make matter convenient to study, in particular its interactions with light. Even a black hole, which emits no light, blocks light by sucking it in — but what if the light coming from behind a black hole simply passed right through, and on into our telescope lenses? How would we ever have proven the existence of a black hole, in that case?

The Cryogenic Dark Matter Search.

That’s the situation physicists face with dark matter. Dark matter does not seem to interact with the universal electromagnetic field in the slightest — that is, it does not absorb or emit light of any kind. In fact, dark matter seems only to interact with the universe as we can observe it through a single physical force: gravity. So, in the case of our invisible black hole, we might have been able to notice it by seeing how light coming to us from a certain section of sky was bent relative to our expectations, knocked slightly off course by passing close to an object bending the surface of the spacetime it’s traversing. Adding up enough light-bending observations, scientists could probably figure out the position and even mass of the invisible singularity.

However, dark matter is harder to study than even that, because it does not come conveniently clumped into super-dense balls like stars and black holes — that would be far too easy. Instead, the primary theory of dark matter says that it is made of hypothetical particles called Weakly Interacting Massive Particles (WIMPs), which are about as well understood as their catch-all name implies. WIMPs don’t even seem to interact with each other through anything more than gravity, meaning dark matter does not fuse to form larger or more complex molecules, and remains in a simple and highly diffuse gas-like state.

Thus, dark matter’s gravitational impact is extremely spread out and, it turns out, can only be observed when we look at the large-scale distribution of visible matter in the universe — things like galactic super-clusters, and the corresponding super-voids. It’s theorized that after the Bing Bang, the properties of dark matter would have led it to settle down far more quickly than regular matter, going from a totally uniform gas-cloud to a somewhat clumped network of smaller clouds and connecting tendrils. These tendrils can stretch across the universe; the distribution of dark matter soon after the Big Bang is thought to have directed where regular matter eventually collected, and thus where and how galaxies formed.

A simulation of the distribution of dark matter, made and rendered by a supercomputer.

So, not only is it invisible, but the effects of dark matter’s gravitational potential are so physically sprawling that they’re hard to measure. The light from a single star won’t be measurably bent by dark matter in reaching us, as it was in passing our invisible black hole; that light might very well have originated, travelled through, and arrived all within the reach of a single universal super-thread of invisible dark matter. So: how did physicists come up with the idea of dark matter in the first place?

The answer is that gravity affects everything, at all scales, according to the same basic formulae. So, scientists started to notice that as they took at larger and larger-scale looks at the universe, these gravity formulae delivered increasingly wrong predictions. As early as the 1930s, Fitz Zwicky discovered that galaxies in the Coma cluster were moving as though they were subject to far more gravitational force than could be explained through a simple accounting of the normal matter we could see. Decades later, Vera Rubin famously noted that stars in spiral galaxies rotate around the galactic center far faster than they ought to, leading to later studies showing that spiral galaxies must be made up of about six times as much dark mass as the regular kind.

A map of the universal Cosmic Microwave Background radiation.

But the really compelling evidence didn’t come about until the advent of techniques like weak gravitational lensing, and the ability to read the cosmic microwave background (CMB) radiation. Gravitational lensing allows a super, super, super large-scale version of watching light bend around our invisible black hole. It gets around the scale issue with… more scale, watching how the collected light from billions of clustered stars bends as it travels across large fractions of the diameter of the known universe. And a number of increasingly accurate CMB maps made between the 1960’s and the 2000’s confirmed similar discrepancies in the movement of mass early in the history of the universe.

Direct observation of WIMPs has been attempted, but never confirmed. In 2009, the Cryogenic Dark Matter Search published evidence of direct observation of dark matter, but the results are not definitive. All the evidence says right now is that something very much like the modern conception of dark matter has to exist.

Calculations of exactly how much of this something would be necessary to create the observed discrepancies have produced some… impressive figures. By modern estimates, the universe is only about 5% regular matter and energy, and about 27% dark matter, or more than five times as much. The remaining 68% of the universe is thought to be dark energy — a topic for another day. The point is that our universe hasn’t just been adjusted by the impact of dark matter, it’s been defined by that impact. The Milky Way is what and where the Milky Way is, due to the early gravitational influence of dark matter.

An upgraded LHC is our best bet to understand dark matter.

Of course, things are turning out to be slightly more complex than described above. Just months ago, one team announced that dark matter may have been observed to interact with itself in some way during an enormous multi-galaxy collision event. This could imply a much more rich sort of dark physics, perhaps even so far as to create some sort of dark chemistry! Some physicists use the phrase “dark world,” or even “dark sector,” to describe this super large-scale alien universe that seems to exist almost in parallel to our own.

The most likely candidate to produce further insight into dark matter is the Large Hadron Collider, which recently reopened after significant power upgrades. With experimental energies now exceeding 13 tera-electron volts (TeV), the new and improved LHC might just be able to smash particles together violently enough to provide real insight into WIMPs, or perhaps even disprove their existence. Finding dark matter was one of the main motivations for the upgrades; it’s an important area of study in physics, as astronomers continue to produce evidence that our world is only a fraction of creation.

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